1. Field of the Invention
The present disclosure relates to image forming apparatuses configured to form an image using an electrophotographic method. In particular, the present disclosure relates to image forming apparatuses such as copying machines, printers, facsimiles, or multifunction peripherals including these multiple functions.
2. Description of the Related Art
In general, a two-component developer containing toner particles and carrier particles as main components is widely used in a developing device included in an electrophotographic or electrostatic-recording image forming apparatus. Especially in electrophotographic image forming apparatuses configured to form full color images and multi-color images, many developing devices use a two-component developer from a viewpoint of image tints.
As well known, the toner density of a two-component developer, i.e., the ratio of the weight of toner particles to the total weight of carrier particles and the toner particles, is a significantly important factor for stabilization of image quality. Toner particles of a developer are consumed during the development to change the toner density. Thus, it is necessary to accurately detect the toner density of the developer at appropriate timings and execute toner replenishment in response to a change in the toner density by use of an automatic toner replenishment control device (ATR), so that the toner density is constantly controlled within an appropriate range to maintain image quality.
Various types of toner density detection units configured to detect the density of toner in a developing container have been available to correct a change in the toner density in the developing device that results from development, i.e., to control the amount of toner to be replenished to the developing device.
For example, there is an optical toner density detection unit configured to detect the toner density using the phenomenon that the reflectance at the time when light is applied to a developer conveyed onto a developer bearing member (hereinafter “developing sleeve”) varies depending on the toner density. Further, a toner density detection unit of an inductance detection method is also used in which the density of toner in a developing device is detected based on a detection signal from an inductance head, which detects the apparent magnetic permeability due to the blend ratio of magnetic carrier to non-magnetic toner of the developer and converts the detected apparent magnetic permeability into an electric signal. Another method that is widely used is a method in which a reference toner image is developed on an image bearing member at a predetermined timing and the density of the reference toner image is detected by an optical density detection unit, which is arranged to face the image bearing member or an intermediate transfer member, to control the toner density of a developer in a developing unit based on the detection result.
When the foregoing toner density detection unit or image density detection unit is used to control the toner density, an initialization operation needs to be executed immediately after a new developing device is installed into a main body of an image forming apparatus. The initialization operation is an operation to determine an output voltage for use as a control target by storing an output of the toner density detection unit or the image density detection unit in a state in which a developer is a new developing device and, thus, the toner density is known in advance. Thereafter, the amount of toner replenishment to the developing device is controlled to attain the target output voltage determined through the initialization operation (refer to, for example, Japanese Patent Application Laid-Open No. 2000-56639).
There are mainly two timings of execution of the initialization operation of a developing device, which are when the main body of the image forming apparatus is installed and when the developing device comes to the end of its endurance life to be exchanged for a new developing device. When the developing device comes to the end of its endurance life to be exchanged for a new developing device, only the developing device that comes to the end of its endurance life is exchanged in general. Thus, the initialization operation is executed only with respect to the exchanged developing device. During the initialization operation, an idle rotation operation needs to be executed to stabilize the height of the surface of the developer, or a development operation needs to be executed to determine the control target voltage for the toner density detection unit or the image density detection unit. Thus, a motor needs to drive the developing device during the initialization operation.
As to a developing device driving configuration, configuration in which a single common motor drives a plurality of developing devices have been suggested and implemented as well as a configuration in which a single motor drives a single developing device, i.e., developing devices of four colors are driven by respective four separate motors. In this case, for example, yellow, magenta, and cyan developing devices are driven by a single common motor, and a black developing device is driven by another motor. This configuration allows the number of motors to be decreased from four to two, so that the costs of the image forming apparatus can be reduced considerably. Furthermore, the space occupied by the motors can be decreased to reduce the size of the image forming apparatus.
However, problems arise when the initialization operation of a specific developing device among a plurality of developing devices driven by a single common motor is executed. The following describes the problems in a case in which, for example, only a yellow developing device among yellow, magenta, and cyan developing devices driven by a single common motor is exchanged and the initialization operation of the yellow developing device is executed.
When the initialization operation of the yellow developing device is executed, the developing device needs to be driven by the motor as described above. At this time, since the motor used to drive the yellow developing device is common to a motor that drives each of the magenta and cyan developing devices, the magenta and cyan developing devices are also driven at the same time although the initialization operation is not necessary for the magenta and cyan developing devices. Consequently, the magenta and cyan developing devices continue to be in an idle rotation state during the initialization operation of the yellow developing device. As a result, an external additive on a toner surface is embedded to cause deterioration, resulting in a significant decrease in developing performance and transfer property. In such a case, malfunctions such as decreased density and increased roughness of magenta and cyan images may suddenly arise after the initialization operation of the yellow developing device.
Further, a toner discharge control has been implemented to prevent problems in images arising from toner deterioration. In the toner discharge control, a predetermined amount of toner is discharged from a developing device during a non-image forming period and, at the same time, substantially the same amount of toner is replenished to replace the toner. In the conventional control method, the video count amount of printed images and the driving time of the developing device are respectively integrated, and whether the toner discharge control can be executed is determined based on the integrated values. Thus, when the driving time of each of the magenta and cyan developing devices, which are driven concurrently with the initialization operation of the yellow developing device, is integrated, the toner discharge control of the magenta and cyan developing devices may be executed immediately after the initialization operation of the yellow developing device is finished. In this case, a problem arises that downtime occurs immediately after the replacement of the developing device with a new developing device to decrease productivity.